Single-phase Two-winding Transformer Model Considering The Gradually Saturating Characteristic Of The Iron Core

The power transformer is one of the most crucial electrical devices of power systems and directly affects the stability of entire system.Low-frequency electromagnetic transients,such as inrush current and DC bias,are the most important electromagnetic physics phenomena of transformers.The influence factors are saturation,eddy current,hysteresis,etc.Among them,core saturation characteristics are the primary factor.Establishing the mathematical model of transformers to simulate its transients is an important way to study transformers.The defect of conventional T and ? models is: saturation levels of each part are same when the core goes into deep saturation,which does not conform to the actual situation.Although this problem is resolved in the conventional ? model,no ? model that characterizes physical processes of the iron core from saturation to deep saturation has been presented.In this paper,the principle of magnetic and electric circuit duality is used to derive the topology-correct ? model for single-phase transformer.The methods to test deep saturation characteristics of the iron core and to assign parameters are proposed.The actual transformers are modeled.Inrush current and DC bias are used to verify the accuracy of the ? model.The calculation and distribution methods of saturation parameters in this paper overcome the defects that the existing ? model cannot describe the core saturation process in detail.The model has high precision for simulating the low-frequency electromagnetic transients of transformer such as inrush current.It provides a fundamental transformer model for EMTP-type electromagnetic transient programs.And this model is commonly used for single-phase two-winding transformers and single-phase potential transformers.The main work of the thesis is as follows:First,the magnetic field distribution of the single-phase two-winding transformer core under different saturation conditions is simulated by COMSOL Multiphysics finite element software.The results show that there are significant differences in the magnetic induction between the core and the side columns after the core is saturated.The principle of transformer magnetic and electric circuit duality is deduced,and the rules of electromagnetic dual transformation and the relationships between components are obtained.According to the physical structure of single-phase two-winding core-and shell-type transformers and single-phase potential transformer,the electromagnetic dual ? model is derived.The reason why the ? model is more accurate than the T model to simulate the low-frequency electromagnetic transients is analyzed.Then,the test process of the ? model parameters is presented.The principle that the transformer core is excited into deep saturation by a DC power source is analyzed.Two methods to measure the incremental inductances of magnetizing branches by an AC-DC hybrid power supply or two independent power sources are proposed.The correctness of the method is validated by simulation on the ATP-EMTP platform.The method of converting the incremental inductance into flux-current data and distributing the data to two magnetizing branches of ? model is put forward.Combined with the non-hysteresis magnetization curve of iron core in the unsaturated region,the entire magnetization curve is obtained.Therefore the ? model considering deep saturation characteristics of the iron core is established.Finally,several single-phase transformers are modeled and simulated based on above methods.Four transformers with different core materials,which are oriented silicon steel,non-oriented silicon steel,amorphous alloy and nanocrystalline alloy,are tested to obtain parameters in the unsaturated and saturated region.The difference of magnetizing characteristics for these four transformers is analyzed.Then results of the inrush current and DC bias by tests and simulations are compared.It shows that errors of T,? and ? models are large when the core saturation characteristics are not considered;errors decrease when one deep saturated data point is included,but the accuracy still cannot meet the engineering requirement.The error of the ? model,in which multiple points are applied to describe the gradually saturating process of the core,is less than 7.9%.In addition,the 10 kV single-phase potential transformer is modeled,and the accuracy of the model is verified by inrush current.